1. Field of the Invention
The present invention relates to a lens control apparatus used in, e.g., a video camera and having both an automatic focus adjustment function and a manual focus adjustment function and, more particularly, to a lens control apparatus, which can automatically switchto the manual focus adjustment mode even in a photographing operation in the automatic focus adjustment mode.
2. Related Background Art
FIG. 1 is a view showing a lens system of a conventional inner focus type lens control apparatus.
In this lens system, a stationary lens group 101, a zoom lens group 102 for performing a zooming (variable magnification) operation, an aperture 103, a stationary lens group 104, a focus compensation lens group 105 housed inside a lens barrel, and having both a focus adjustment function and a compensator function (a function of correcting movement of a focal plane upon zooming), and an image pickup element 106 are arranged along the optical axis from a direction of an object on the left side in FIG. 1. The image pickup element 106 has an image pickup surface 106a.
In a so-called front-element focus type lens system for performing focus adjustment using a movable lens group in place of the stationary lens group 101, when manual focus adjustment is to be performed, a helicoid or the like fitted on a lens barrel is rotated, and its drive force is mechanically supplied to a lens, thereby moving a focus adjustment lens along the optical axis. However, in the inner focus type lens system shown in FIG. 1, it is difficult to arrange such a mechanical and external drive means for the following reasons.
More specifically, since the lens group for focus adjustment is arranged inside the lens barrel, it is difficult to arrange a helicoid, a knob, and the like on the outer surface of the lens barrel. In addition, holes for the helicoid, knob, and the like must be formed on the outer surface o the product covering the entire lens barrel, and it is not preferable in terms of the outer appearance and cost.
Since the focus compensation lens group 105 has both the compensator function and the focus adjustment function, the in-focus position of the lens group 105 on the image pickup surface 106a varies depending on the object distance and the focal length even if the focal length remains the same. When the object distance is changed at each focal length, the in-focus position of the lens group 105 on the image pickup surface 106a is continuously plotted, as shown in FIG. 2. In a zooming operation, one of paths shown in FIG. 2 is selected according to the object distance, and the lens group 105 is moved along the selected path, thus attaining the zooming operation free from an out-of-focus state. In order to realize tracing of the path, as described above, the position of the zoom lens group 102 and the object distance are detected by certain methods to determine one of the plurality of paths shown in FIG. 2 on which the lens group 105 is present. For example, as shown in FIG. 3, each path according to the zoom lens position and the focus compensation lens position is divided into a plurality of blocks, and tables storing drive speeds capable of tracing the paths corresponding to the positions are formed in units of blocks. Thus, the focus compensation lens group 105 must be moved to the next position, i.e., block using the path tracing drive speed set for the selected block according to the zoom lens position and the focus compensation lens position. In general, the drive control of the lens group 105 is finely performed using a microcomputer and a stepping motor. When a stepping motor is used, the position of the lens group 105 is detected by counting the number of drive steps in place of using a position encoder in the lens group 105.
Therefore, when a drive command or a drive force is applied to the lens group 105 from another system different from the microcomputer, the microcomputer cannot detect the accurate position of the lens group 105, and may erroneously detect an abnormal drive state of the lens.
However, many photographers want to perform manual focus adjustment during a photographing operation. Furthermore, in consideration of the fact that the performance of the automatic focus adjustment (auto focus) function of currently commercially available apparatuses is not perfect depending on the photographing state, the manual focus adjustment (manual focus) function is also indispensable in the inner focus type lens system.
When a manual focus adjustment is performed in the inner focus type lens system, the following two operations are possible. 1 For example, the manual focus adjustment mode is selected using an auto/manual focus selection switch for selecting the automatic or manual focus adjustment function, and then, a drive command switch in a direction of shortest range or in a direction of .infin. is depressed, thus performing a focusing operation through the microcomputer. 2 A rotary ring having an encoder function capable of detecting the rotational direction and the rotational amount per unit time is fitted on the lens barrel, the rotational amount and direction of the ring are read by the microcomputer, and the focus compensation lens group 105 is driven according to the read result.
However, since the focus adjustment lens group can only be driven through the microcomputer, and the focusing operation is performed after the manual mode is selected, the conventional system has the following drawbacks.
1 When the manual focus mode is to be selected during a photographing operation, a photographer must fumble for the auto/manual focus selection switch, resulting in camera-shake since he or she removes his or her eye from the finder, or in a long fumbling time.
2 When an object which cannot be easily focused by the auto focus function cannot be focused in a photographing operation in the auto focus mode, it is not easy to select the manual operation while continuing the photographing operation for the reason 1.
3 Once the manual focus mode is selected, the auto focus mode cannot be automatically resumed unless a reset operation (e.g., a power-OFF operation) is performed. Therefore, a mode selection operation must be performed when a photographing operation using the auto focus mode is to be performed in a shot next to the photographing operation in the manual focus mode, resulting in troublesome operations.
4 When a photographer forgets that the manual focus mode is selected, an in-focus state can no longer be obtained if he or she directs the camera to an object having a different distance.